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Building an 8" Closed Tube Reflector

Construction Pictures
Adding the Focuser and Spider

This schematic drawing is used in laying out the focuser and the spider. One does not want to drill multiple holes all over the place and then spend time filling them in and re-testing the telescope on the stars, not to mention drilling wrong holes in a $1,000 carbon fiber tube. The laying out the these parts can be accomplished in a very systematic and fool-proof way. After building the spider (see building the spider) measure the distance from the vane mounting holes to the center point of the diagonal, allowing about 3/4" of outward (toward the open end of the tube) travel before the holder backs into the spider. This gives you sufficient adjustment room. This dimension is known as FSD, focuser to spider distance.

Next, we measure the distance from the center face of the mirror, the vertex, to the inner face of the end cap. This dimension is the MID, mirror internal distance, or the amount the mirror face extends into the tube from the end of the tube. With telescope it will be about 2.375" Finally, we take the distance from the base of the focuser to the top of the focuser barrel when wracked half way out, FH, or focuser height. You might think of putting an additional 1/2" spacer place under the focuser to allow for some future need to in focus. With the moonlight focuser and a curved tube plate and a 1/2" spacer plate, this is 3.25". The only other dimension is one half of the outer tube diameter, HTD, in this case about 4.95". Going back to the schematic drawing above we take the focal length, add the MID, subtract the HTD and subtract the FH. The  result is the distance from the back end of the tube (without the end cap on, to the center of the focuser tube hole, about 42.175". Add the FSD and you get the distance from the focuser hole to the spider vane mounting hole. I suggest doing everything from scratch about four times (as I do) and making certain you haven't missed something (as I can do). If you are careful, it works out

The picture above shows the spider with about 3/4" of adjustment remaining. The picture to the right shows the central position of the focuser and the longitudinal position of the spider vane holes.

The best way to place the three vane holes is the cut out a piece of cardboard the diameter of the tube and mark it off with a compass with three 120 degree points. Make marks on the side of the tube corresponding to the three points and, setting the tube on its back end and using a square held against the top end, draw lines downward to approximately the focuser position as seen in the picture below.

The picture above shows the appropriate markings and the focuser hole drilled out with a 2.5" hole saw.

Touching up the holes with cyanoacrylate glue solidifies thing and makes for a cleaner job after sanding and cleaning away ragged ends.

Test the curved saddle position. Take note that the saddle is not symetrical and extends further over on one side.

The spider can nor be fitted and adjusted and centered (see building the spider).

Final fitting of the focuser after painting.